1. Field of the Invention
This invention relates to communication systems and particularly to such systems for use in industrial control applications. More particularly, the invention relates to a ring-configured, network communication system that continues uninterrupted communication even in the event of a failure in communication lines.
2. Background
Industrial control systems are used in industrial processes to control pressures, temperatures, mass transfers, and other process parameters. Control systems may include many input/output devices such as temperature sensors, pressure sensors, pressure regulators, and other similar devices positioned at various locations in the process equipment being controlled. Modern control systems also include one or more programmable logic controllers (PLCs) for controlling various input/output devices in the system, based upon data collected from the input/output devices and based upon instructions programmed on the controller.
These input/output devices and PLCs must communicate with each other in order for the control system to provide the desired control for the process. Both data and instructions must be communicated between devices in the control system. Thus, an industrial control system requires a communication arrangement to enable the various control system devices to communicate with each other.
Some industrial processes may require very precise control. Failure of communications between the various elements of an industrial control system may upset the entire process and have dire results. For example, failure of process control communications in a chemical manufacturing process may result in the production of an entirely different chemical than what was intended. Thus, a communications failure in an industrial control system may require that the entire process being controlled be aborted or at least suspended until communication is reestablished. Also, failure of communication and control in some processes may pose very serious safety risks. Therefore, the communication systems employed in industrial control systems must be very robust.
It is a difficulty in this situation that communication lines required between various elements of an industrial control system must commonly traverse harsh environments or areas of high activity. In these areas, there is a constant danger of damage to the communication lines resulting in a loss of communication in the control system. Also, conditions in some areas of an industrial process may interfere with certain types of transmissions. For example, electrical noise in certain areas of a plant may interfere with electrical transmissions.
To deal with these situations and others, data and instructions may be converted to optical signals for transmission between the various control system devices through optical fibers. Also, the various elements of an industrial control system may be connected for communications in a ring configuration that allows communications in both directions around the ring. In this closed ring configuration, a single break in the ring does not cause a loss of communications since communication is still possible in the opposite direction from the location of the break. Closed communications rings, however, raise the problem of signal oscillation, which interferes with communications.
One way to solve the problem of ensuring communications in the event of a failure along a communication path in an industrial process control system is to provide multiple communication paths between the various elements of the system. In the event of a failure on one communication path, communications are still maintained along a secondary path. Since uncontrolled duplicate messages in a multi-path network can give rise to a message storm, which continually delivers redundant messages and locks up the network, great care must be taken in this situation.
Still further complications arise due to the nature of communications in an industrial control system. While some communications of data and instructions may be relatively continuous in some process control applications, communications may be infrequent in other control systems. In situations in which long gaps appear between data or instructions, a communication failure may not be readily apparent. That is, a system that monitors for communication failures may mistake a communication fault for a gap in data, or may mistake a gap in data for a communication fault. Thus detecting a failure in the communication process presents difficulties in an industrial control system.
In one system described in U.S. Pat. No. 6,307,652 issued Oct. 23, 2001, which is hereby incorporated herein by reference, a “control unit” is included in a duplex optical communication ring not to interface a peripheral device to the ring, but to operate, at least in some respects, as a master unit to control aspects of communication within the ring, which includes initially blocking data in both paths of the ring, i.e., in both the send and receive directions of a transmit and receive port of the control unit, which are connected to respective fiber links. Data units are included in the optical ring and are operable both to interface respective control or data acquisition system devices to the ring and also to control aspects of communication within the ring, i.e., as slaves to the control unit in at least some respects. Responsive to detection of a break in a fiber link, the system moves the location of the blocking. That is, if one of the data units detects a break, it responsively blocks data at the location of the break and signals the control unit to stop blocking.